1. Field of the Invention
The present invention generally relates to a display apparatus for displaying characters and figures (hereinafter referred to as “image”) in a computer system. More specifically, the present invention is directed to a display apparatus equipped with a screen saver function in the computer system.
2. Description of the Prior Art
FIG. 6 (PRIOR ART) (schematically represents a display system employed in a so-called “DOS/V” type personal computer system. This display system displays character data (text data), and image data (graphic data). It should be noted in this specification that a term “image” apparently contains images, figures, characters, graphics and the like. For example, image data may correspond to character data itself.
In this drawing, reference numeral 100 indicates a CRT display apparatus. This CRT display apparatus 100 is connected to a computer main frame, and may display thereon the text data and the graphic data. In this computer main frame, there are provided a central processing unit (CPU) 101, a graphic controller 102, a video sequencer 103, a video RAM (VRAM) 104, an attribute controller 105, a video DAC 106, and a CRT controller 107.
The VRAM 104 is such a memory for temporarily storing data used to display dots on a display screen. Image data processed by the CPU 101 is temporarily stored into this VRAM 104. The VRAM 104 is so arranged that for instance, a plurality of video planes having capacities of 64 KB (kilobytes), or 128 KB are combined with each other.
The graphic controller 102 sequentially writes the image data processed by the CPU 101 into the VRAM 104 in accordance with the image sequence defined by the sequencer 103. Since all of the video planes of the VRAM 104 are rewritten by the image data within one time, this graphic controller 102 executes a logic calculation between the data supplied from the CPU 101 and the display screen, and thus can draw images at a high speed. In the text mode, the graphic controller 102 converts the data outputted from the VRAM 104 into parallel digital data, whereas in the graphic mode, the graphic controller 102 converts the data derived from the VRAM 104 into serial digital data, and then outputs either the parallel or the serial digital data to the video DAC 106.
The attribute controller 105 contains a palette register used to produce a 6-bit color code from 4-bit display data outputted from the VRAM 104 and to output this 6-bit color code.
The video DAC 106 converts the image data (digital data) supplied via the attribute controller 105 into an analog signal to be displayed on the display 100.
The CRT controller 107 generates a vertical scanning signal and a horizontal scanning signal used in the display 100, and further generates a read address used to read the image data from the VRAM 104. Then, in accordance with the image display sequence defined by the sequencer 103, the image data outputted from the VRAM 104 is displayed on the display 100 under control of the CRT controller 107.
The foregoing description is made of a basic arrangement VGA (Video Graphics Array) used to display a screen image. Now, a sequential operation of displaying a Japanese language on such a DOS/V type personal computer will be described as a concrete example. In this case, text data is displayed on the CRT display 100.
First, when this DOS/V type personal computer is initiated, a predetermined storage area is secured in an extended memory, and a Japanese character font is stored into this extended memory area.
On the other hand, a virtual VRAM (namely, virtual text VRAM) area is secured in a conventionary memory. This conventionary memory implies the most major memory area having the capacity of 640 KB within the main memory device. When a Japanese character is entered from a keyboard, a code of this inputted character is written into this virtual VRAM.
Subsequently, a Japanese character font corresponding to the character code written into this virtual VRAM is received, and then the received Japanese character font is written into the graphic VRAM (namely, above-described video memory) to be represented on the display 100.
Such a font representation is carried out by a wordprocessor operated, or accessed on this DOS/V type personal computer. There are many possibilities that the personal computer is not actually operated for a predetermined time period, because a computer operator takes a rest while this operator forms a document, or this computer operator interrupts this document forming work, so that entries of the Japanese characters are stopped.
In such a case, if the same content of the display screen is continuously represented for a long time duration, then a so-called “image sticking” phenomenon may occur. To avoid this difficulty, a so-termed “screen saver” program has been developed. In accordance with this screen saver program, when a computer is not actually operated for a predetermined time period, screen protecting image data is slightly moved on a display screen.
However, since even such a screen saver program constitutes a portion of a computer program, this screen saver program is loaded on a main memory of a central processing unit (CPU) employed in a computer. Then, this screen saver program is executed by this CPU, and thus, the screen protecting image data is displayed on a display unit under control of a graphic controller and a CRT controller.
As a result, even when the conventional computer is not actually operated by operators, the central processing unit and the respective controllers must be actually operated, so that the electric power is continuously consumed.
This power consumption causes a very serious problem in a portable notebook type personal computer, namely a battery capacity thereof is limited.
On the other hand, a certain type of portable notebook personal computers is equipped with a low power consumption mode. In this low power consumption mode, a screen image representation is stopped when this type of personal computer is not actually operated within a preselected time duration.
In this stopping condition of the screen display, the above-explained “image sticking” phenomenon may be avoided even when a such a screen saver program is not introduced. However, since the screen display is stopped, no operator can confirm as to whether or not the notebook type personal computer itself is operated under initiation condition, resulting in inconvenient operation.